American Citizen on Mon, 14 Jul 2025 22:11:59 +0200


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question on solutions of rational versus integer ternary quadratics


Hello:

I am working with the following conic and 2 ternary quadratics

conic =  [261/5, -3481/50, 261/5, -1739/100, -1739/100, -1]
T9Q([x,y,z]) = 261/5*x^2 + (-3481/50*y - 1739/100*z)*x + (261/5*y^2 - 1739/100*z*y - z^2) T9Z([x,y,z]) = 5220*x^2 + (-6962*y - 1739*z)*x + (5220*y^2 - 1739*z*y - 100*z^2)

Over the range of integers x,y,z, for -10,000 <= x,y,z <= 10,000 I found 2367 solutions for T9Q, but only 1076 for T9Z. However the 1291 solutions from T9Q plugged into T9Z do work.

Why is T9Q missing 1,291 solutions?

Randall

btw: These ternary quadratic solutions collapse to just 113 points on the conic, but the [0,0] point has to be removed.

Another btw note: I had special rational points on conics, it takes 2 rational points to successfully recover all rationals on the conic, using the point-slope method to parameterize, and the reason is simple, for y - y1 = m(x-x1) where [x1,y1] is known and m is the slope, if you set m=0 for slope zero, the equation collapses to y - y1 = 0, and that only recovers your original [x1,y1] point and misses the 2nd y point which IS on the conic. I had to use 2 points and only found this out after laboring on this for at least 3 days wondering why my rational lattice points were missing some rationals.